Stator assembly for bounding the flow path of a gas turbine engine

ABSTRACT

A stator assembly 20 for a gas turbine engine is disclosed. The stator assembly includes an outer case 22 and an array of stator vanes 32 extending inwardly across an annular flow path 18 for working medium gases. The stator assembly includes a duct 34 which engages the stator vanes and which is trapped between a stator vane and a stator element 38 extending inwardly from the outer case. The duct includes a transition piece 78 which extends axially to shield a portion of the stator vane. In one embodiment the duct includes an extension 80 which extends into proximity with an outer air seal assembly 31 to form a cooling air chamber 36 adjacent to the stator element.

DESCRIPTION

1. Technical Field

This invention relates to gas turbine engines and more particularly to astator assembly adjacent to the flow path for working medium gases ofsuch an engine. This invention was developed in the field of aircraftgas turbine engines and has application to turbine engines in otherfields that employ arrays of stator vanes to direct a working mediumfluid.

2. Background Art

An example of a gas turbine engine of the type to which the presentinvention applies is a gas turbine engine having a compression section,a combustion section and a turbine section. An annular flow path forworking medium gases extends axially through such an engine. A rotorassembly extends axially through the compression and turbine sectionsand is circumscribed by a stator assembly. The stator assembly includesan engine case. Rows of rotor blades extend outwardly from the rotoracross the working medium flow path in both the turbine and compressionsections. An array of stator vanes extends inwardly from the engine caseacross the working medium flow path at the downstream end of most bladerows for directing the working medium gases of the engine into the nextworking station of the engine.

Examples of engine structures employing rotor blades and stator vanesinwardly of an outer case are shown in: U.S. Pat. No. 3,966,354 issuedto Patterson entitled "THERMAL ACTUATED VALVE FOR CLEARANCE CONTROL";U.S. Pat. No. 3,992,126 issued to Brown et al. entitled "TURBINECOOLING"; and U.S. Pat. No. 4,011,718 issued to Asplund entitled "GASTURBINE CONSTRUCTION".

As shown in these patents, the combustion section of the engine is usedto burn fuel in the engine to add energy to the working medium gases. Asthe hot, working medium gases are expanded through the turbine sectionof the engine, the arrays of rotor blades and stator vanes immediatelydownstream of the combustion section are bathed in the working mediumgases. In this region of the engine, cooling air is flowed inwardly ofthe outer case to the interior of the stator vanes to maintain thetemperature of the stator vanes within acceptable levels.

At some location downstream of the cooled arrays of stator vanes coolingair is not flowed to the interior of the vanes. An example of such aconstruction is shown in U.S. Pat. No. 3,644,057 issued to Steinbargerentitled "LOCKING DEVICE". Even though the stator vanes are not cooledinternally, efforts are being made to ensure that the stator vanes andassociated components are not unacceptably heated by the hot workingmedium gases. Accordingly, scientists and engineers are working todevelop a stator assembly employing stator vanes which avoids theunacceptable heating of the stator vanes and components associated withthe stator vanes.

DISCLOSURE OF INVENTION

According to the present invention, a gas turbine engine having aworking medium flow path, an outer case extending about the flow path,and an array of stator vanes supported from the outer case, includes aduct that engages the array of stator vanes at one location, engages thestator vanes and the case at another location and extends between thelocations to shield the array of stator vanes from the working mediumgases.

In accordance with one embodiment, the duct is formed of arcuatesegments each having a cantilevered extension which extends intoproximity with an adjacent stator element to form a cooling air cavityadjacent to the engagement between the case, the vanes and the duct.

A primary feature of the present invention is a stator structure for agas turbine engine. The stator structure includes a duct formed of aplurality of circumferentially extending segments. The duct has anaxially extending portion which bounds the working medium flow path.Another feature is an array of stator vanes. Each stator vane has atleast one airfoil which extends across the working medium flow path.Each vane engages the outer case. The duct engages the stator vanes atone location and is trapped between the stator vanes and the case atanother location. In one embodiment, the duct has a cantileveredextension. The extension extends axially into proximity with an adjacentstator structure to form a cooling air chamber adjacent to theattachment of the duct to the case. The vanes are spaced from the outercase to form a second cooling chamber. A plurality of holes extendthrough the outer case adjacent to the attachment to provide a flowpassage for cooling air through the slots from one cooling air chamberto the other. In one embodiment, each airfoil has a leading edge, atrailing edge and a chordwise dimension L. The length from the leadingedge to the attachment at the case is a distance L' which is greaterthan or equal to the distance L (L'>L).

A primary advantage of the present invention is the fatigue and creeplife of the stator structure at the attachment of the vane to the casewhich results from interfering with the transfer of heat from theworking medium gases through the airfoils of the vanes to the casing.Another advantage is the aerodynamic efficiency which results fromdefining the working medium flow path with an axially extending duct.Still another advantage is the interchangeability of the high pressureturbine with the low pressure turbine which results from having a ductwhich is easily replaced with another duct having a different contourbut which is adapted to engage the vanes of the low turbine and the casein the same way that the replaced duct engages the vane and the case.

The foregoing features and advantages of the present invention willbecome more apparent in the light of the following detailed descriptionof the best mode for carrying out the invention and in the accompanyingdrawing.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a side elevation view of an axial flow, gas turbine enginewith a portion of the outer case broken away to show a portion of theturbine section.

FIG. 2 is an enlarged cross-sectional view of the portion of the turbinesection shown in FIG. 1.

FIG. 3 is a partial perspective view of a portion of the turbine sectionof the engine.

FIG. 4 is an alternate embodiment of the flange shown in FIG. 2.

BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 1 shows an axial flow, gas turbine engine 10 which has an axis ofrotation R. A portion of the engine is broken away for clarity. Theengine has a compression section 12, a combustion section 14 and aturbine section 16. An annular flow path 18 for working medium gasesextends axially through the sections of the engines. A stator assembly20 extends axially through these sections to bound the working mediumflow path. The stator assembly includes an outer case 22 which extendscircumferentially about the working medium flow path.

A plurality of cooling air tubes 24 extends circumferentially about theouter case 22 in the turbine section 16. The cooling air tubes are inflow communication with a source of cooling air such as the compressionsection 12. The tubes are adapted to impinge cooling air on the outercase during preselected operating conditions of the engine to adjustinternal operating clearances in the turbine section.

The turbine section 16 includes a high pressure turbine 26 and a lowpressure turbine 28. An array of rotor blades in the high pressureturbine, as represented by the single rotor blade 30, extends outwardlyacross the working medium flow path. An outer air seal assembly 31 isspaced radially from the array of rotor blades and is attached to theouter case 22. In the low pressure turbine, the stator assembly includesan array of stator vanes as represented by the single stator vane 32. Aduct formed of a plurality of arcuate duct segments, as represented bythe single segment 34, extends between the stator vanes of the lowpressure turbine and the outer air seal of the high pressure turbine.The duct is spaced radially from the outer case leaving a first coolingair chamber 36 therebetween which extends circumferentially about theaxis R. The annular flow path 18 inwardly of the duct includes anannular transition region 37 which extends from the array of rotorblades of the high pressure turbine to the array of stator vanes of thelow pressure turbine.

FIG. 2 is an enlarged cross-sectional view of a portion of the gasturbine engine shown in FIG. 1 showing in more detail elements of thestator assembly 20 such as the outer case 22, the outer air sealassembly 31, the array of stator vanes 32 and the duct 34.

The outer case 22 has a first stator element, such as the first flange38, which extends inwardly from the outer case. The flange is adapted bya groove 40 at a first axial location A₁ to trap the array of statorvanes 32 and the duct 34. A second stator element, such as the secondflange 42, extends inwardly from the outer case. The second flange islocated at a second axial location A₂ which is spaced axially from thefirst location. A third stator element, such as the adjacent outer airseal assembly 31, is spaced axially from the first flange. The outer airseal assembly includes a plurality of arcuate seal segments 44 which arespaced radially from the array of rotor blades. A plurality of arcuaterings 46 extend radially inwardly from the outer case to the arcuateseal segments for supporting the downstream end of the arcuate sealsegments from the outer case.

The array of stator vanes 32 extends circumferentially about the annularflow path 18 for working medium gases. Each stator vane is spacedcircumferentially from the adjacent vane and is spaced radially from theouter case 22 leaving a second chamber 48 for cooling air therebetween.The stator vane has a platform 50 and at least one airfoil 52 extendinginwardly from the platform across the working medium flow path. Theairfoil has a leading edge 54, a trailing edge 56 and a chordwisedimension L extending between the leading edge and the trailing edge.The chordwise dimension L is measured in a direction that isperpendicularly oriented with respect to both the leading edge and thetrailing edge at a location adjacent to the platform. The vane has afirst foot 58 which extends for a distance L' from the airfoil edgenearest the foot, which in this case is the leading edge of the airfoil.The length L' is greater than or equal to the length L (L'≧L).

The first foot 58 of the vane extends axially across the first andsecond cooling chambers 36, 48 to the outer case. The first foot istrapped by the first flange 38 of the outer case. The vane has a secondfoot 60 which is adjacent to the second flange 42 of the outer case.Each second foot has a hole 62. The second flange 42 has a plurality ofbolt holes 64, each of which is circumferentially spaced from anadjacent hole such that the hole 62 in the vane is aligned with anassociated hole in the second flange. These holes adapt the flange andthe vane to receive a fastener such as a bolt and nut combination 66 forsecuring the vane to the second flange.

The vane 32 also includes a groove 68 which is bounded by the platform50 and the first foot of the vane. The groove 68 extendscircumferentially in the array of vanes at a third axial location A₃located axially between the first axial location A₁ and the second axiallocation A₂.

Each arcuate duct segment 34 has an axis of radius R₁ which iscoincident with the axis of rotation R. The duct segment has a concaveside 70 which bounds the transition region of the annular flow path 18and a convex side 72 which is spaced radially from the outer case toform the first circumferentially extending chamber 36 for cooling airtherebetween. The duct segment has a first foot 74 at a first end 75which is radially oriented with respect to the axis R. The first footextends radially inwardly across the first chamber to the outer case 22.The first foot of each duct segment is trapped between an associatedvane 32 of the array and the first flange 38 of the outer case at thefirst axial location. A second foot 76 at a second end 77 is radiallyoriented with respect to the axis R and is spaced axially from the firstfoot by a distance D₁. The second foot on each duct segment extends intothe groove 68 of the vane and is trapped by the vane at the thirdlocation A₃. A transition piece 78 is disposed radially inwardly of thefirst foot. The transition piece is angled with respect to the axis ofradius R₁ and extends axially between the first foot and the second footof the duct. The transition piece has an extension 80 which extends incantilevered fashion for a second distance D₂ from the first foot intoproximity with the outer air seal assembly. The second distance D₂ isequal to or greater than the first distance D₁. The extension has acurved nose 82 which extends axially over the outer air seal assembly.The dotted lines show the relationship between the outer air sealassembly and the duct segment before operation of the gas turbine enginebegins.

A plurality of cooling air holes 83 in the first flange 38 of the outercase 22 connects the first cooling air chamber 36 (which is between theduct 34 and the outer case 22) with the second chamber 48 (which isbetween the array of stator vanes 32 and the outer case). The firstcooling air chamber is divided by the first foot 74 of the duct into anupstream chamber 36_(u) and a downstream chamber 36_(d). The upstreamchamber is bounded by the first foot 74 of the duct, the extension ofthe duct 80, the outer case and the outer air seal assembly 31. Theupstream chamber is a smaller chamber within chamber 36 which isadjacent to the first stator element, that is, first flange 38. Aplurality of holes 84 in the outer air seal assembly adapts the assemblyto place the upstream chamber in flow communication with a source ofhigh pressure cooling air such as the compression section 12. A sealmember 86 extends circumferentially in the second cooling air chamber 48and axially from the first flange 38 at the first axial location A₁ tothe second flange 42 at the second axial location A₂ to provide a sealto the cooling air chamber between the vane and the outer case. An innercooling air chamber 48_(i) is formed between the vanes and the sealmember.

FIG. 3 is a partial perspective view of the outer case 22, the array ofstator vanes 32, and the array of arcuate duct segments 34 shown in FIG.2. The seal member 86 and the second flange 42 are broken away andsectioned to better show the relationship between the second flange andthe second foot 60 of the vanes. As shown in this embodiment, eachstator vane includes three airfoils extending inwardly across theworking medium flow path. Each duct segment is spaced circumferentiallyfrom the adjacent duct segment and is adapted by a groove to receive afeather seal 88. The first foot of each duct segment has at least oneslot 90 and a circumferentially extending opening 92 to reduce the hoopstrength of the segment. Cooling air from the upstream chamber 36_(u) isin flow communication with the downstream chamber 36_(d) through theslots and openings. A small amount of cooling air flows into thedownstream chamber and through the gap between the adjacent vanesegments into the inner chamber 48_(i) formed between thecircumferentially extending seal member 86 and the array of statorvanes. Thus, the first foot of the stator vane and the first statorflange are bathed in cooling air on all sides.

FIG. 4 is an alternate embodiment of the first flange 38 shown in FIG.2. In FIG. 2, the flange and the outer case 22 form a one-piecestructure. In the alternate embodiment, the flange includes acircumferentially extending ring 94 which is integrally attached to theouter case.

During operation of the gas turbine engine, working medium gases areflowed along the annular flow path 18 which extends axially through theengine. The gases are compressed in the compression section 12 and mixedwith fuel in the combustion section 14. The gases and fuel are burnedtogether to add energy to the gases. The hot, high pressure gases areflowed from the combustion section to the turbine section 16 of theengine. The gases are expanded in the turbine section through arrays ofrotor blades 30 and stator vanes 32 to extract useful work from thegases.

As the gases are discharged from the array of rotor blades 30 at thedownstream end of the high pressure turbine 26, the gases are passedthrough the transition region 37. The gases experience a suddenexpansion in the transition region before being passed into the lowpressure turbine 28. Because of the sudden expansion of the workingmedium gases, the velocity of the hot gases decreases and the staticpressure increases creating the potential for significant flow losses ifthe flow path is not contoured to avoid these losses.

Accordingly, the transition piece 78 of the duct 34 is aerodynamicallycontoured to minimize flow losses connected with the sudden expansion.An example of such contouring is the nose 82 of the transition piecewhich is rounded to provide a smooth transition between the flow pathcontour C at the downstream side of the rotor blade 30 and the flow pathcontour C' along the length of the duct and the platform 50 of the vane.

A redesigned low pressure turbine having a different rate of expansionor recontoured vane platforms 50 at the entrance to the low pressureturbine might be employed in place of the low pressure turbine 28 shownin FIG. 2. The duct makes easier such redesigns by providing a separateelement of stator structure which is easily removed and replaced with arecontoured duct to provide a customized transition region that joinsthe old high pressure turbine with the new low pressure turbine. In asimilar fashion, a new high pressure turbine might be joined to an oldlow pressure turbine.

As the hot working medium gases pass through the transition region 37,heat is lost from the gases to the stator assembly 20. In particular,the uncooled airfoils 52 of the array of stator vanes 32 are bathed inthe hot working medium gases. A portion of the heat transferred to theairfoils is transferred through the vanes to the outer case at thepoints of attachment of the vanes to the case.

The point of attachment at the first flange 38 is especially susceptibleto the phenomenon known as creep because heat is transferred through thevanes to the outer case 22 at the flange and because forces exerted bythe working medium gases on the airfoils are transferred at the firstflange to the outer case. Accordingly, several structural elements ofthe stator assembly are designed to limit the transfer of heat from theworking medium gases to the first flange of the case, thereby limitingthe operating temperature of that portion of the case and enhancing itscreep life. The duct 34 is a key element of this design.

The temperature of the working medium gases in the transition region 37is greatest at the entrance to the region and decreases in the axialdirection as the gases expand. As the gases pass through the transitionregion, the extension 80 of the transition piece 78 of the duct extendsupstream of the first flange 38 to shield the first flange 38 from thehot gases and to prevent the direct contact between the gases and theflange.

Heat is transferred from the gases by convection and radiation to thetransition piece 78. The temperature gradient in the transition piecewhich results from this transfer of heat generally follows thetemperature gradient in the gases and decreases in the axially rearwarddirection. The first foot 74 of the duct is spaced by the extension fromthe hot upstream part of the transition piece. This decreases the radialtemperature gradient in the first foot as compared with constructionshaving the first foot secured to the hot part of the transition pieceand decreases the radial conduction of heat from the transition pieceoutwardly to the first flange 38. In addition, as shown in FIG. 3, thefirst foot has elongated openings 92 to decrease the area through whichheat will flow in the radial direction thereby increasing the thermalresistance of the foot to the flow of heat.

Heat which is transferred thought the airfoils 52 to the leading edgeregion 54 of the platform of the vane is conducted toward the firstflange 38 by the first foot 58. The length of the first foot of the vaneincreases the thermal resistance of the vane as compared with thoseconstructions in which the first foot is of shorter length. As comparedwith such constructions, this increased thermal resistance decreases theamount of heat transferred from the leading edge region of the vane tothe first flange 38. In addition, the first foot 74 of the duct 34 andthe first foot 58 of the vane are bathed in the cooling air at theupstream and downstream chambers 36_(u), 36_(d) of the first cooling airchamber 36 and the inner chamber 48_(i) of the second cooling airchamber 48. Finally, the cooling holes 83 which place the first coolingair chamber in flow communication with the second cooling air chamberprovide a plurality of cooling conduits for cooling the base of thefirst flange.

As a result of these measures, the first flange 38 and the region aboutthe first flange on the outer case 22 operate at a much coolertemperature than a similar case not employing measures such as theshielding provided to the first flange, the increase in thermalresistance to conduction of those elements contacting the first flangesuch as the first foot 74 of the duct and the first foot 58 of the vane,the direct cooling of the first flange by the holes 84 extending throughthe first flange and the cooling provided by the first cooling chamber36 and the second cooling chamber 48 to the outer case 22 and elements58,74 conducting heat to the outer case. Because creep is directlyproportional to temperature, the lower temperature increases the creeplife of the outer case around the first flange as compared withconstructions which are unshielded and not cooled by cooling chambersand cooling air holes.

These measures to decrease the case temperature in the region of thefirst flange 38 are so effective that often the temperature in theregion about the first flange is smaller than the case temperature inthe region about the second flange 42. Because of the differences intemperature, differences in radial growth occur. This difference inradial growth causes the vane to rotate slightly about the second flangein the axial direction causing the airfoil to tilt rearwardly by aslight amount. Tolerance variations in the radial height of the firstfoot 58 of the vane and the first foot 74 of the duct also cause a smallamount of vane rotation.

The tilt of the airfoil due to vane rotation is proportional to theradius of rotation of the airfoil that extends from the point ofrotation at the second flange 42 to the first flange 38 which acts toresist the rotation of the vane. In the embodiment shown, the radius ofrotation is equal to the overall length of the vane. The overall lengthis extended by the long first foot of the vane which extends a distanceat least equal to the chordwise dimension L of the airfoil. This longeroverall length decreases the tilt of the airfoil 52 for a giventolerance variation or difference in radial growth as compared withvanes having a shorter overall length. As a result, flow losses whichresult from tilting of the airfoil are decreased and the aerodynamicefficiency of the engine is improved.

Although the invention has been shown and described with respect todetailed embodiments thereof, it should be understood by those skilledin the art that various changes in form and detail thereof may be madewithout departing from the spirit and the scope of the claimedinvention.

I claim:
 1. In a gas turbine of the type having an annular flow path forworking medium gases which extends axially through the engine and astator assembly which extends circumferentially to bound the flow path,the stator assembly including an outer case extending circumferentiallyabout the flow path and an array of stator vanes each of which engages astator element extending inwardly from the outer case, the improvementwhich comprises:a duct bounding the working medium flow path formed of aplurality of arcuate duct segments each of which has a first end whichis trapped between a stator vane of the array of stator vanes of saidarray of stator vanes and the stator element extending inwardly from theouter case and a second end which engages at least one of said vanes,the duct extending inwardly of the stator vane to shield a portion ofthe vane from the hot, working medium gases of the flow path tointerfere with the transfer of heat from the gases to the outer case. 2.The gas turbine engine as claimed in claim 1 wherein the stator elementis a first stator element, wherein the engine has an adjacent statorelement spaced axially from the first stator element and wherein theduct has an extension which extends beyond the first end into proximitywith the adjacent stator element to shield the first stator element fromthe working medium gases.
 3. The gas turbine engine as claimed in claim2 wherein the first end of each segment includes a foot which extendsradially inwardly to be trapped between the first stator element and thevane and wherein each segment has a transition piece having an extensionwhich extends axially beyond the first end and which is cantileveredfrom said end.
 4. The gas turbine engine as claimed in claim 3 whereinthe first stator element is integrally attached to the outer case. 5.The gas turbine engine as claimed in claim 3 wherein the first statorelement and the outer case form a one-piece structure.
 6. For a gasturbine engine which has an axially extending flow path for hot, workingmedium gases, a stator assembly which bounds the working medium flowpath having an outer case extending circumferentially about the flowpath, which comprises:(1) a first stator element extending inwardly fromthe outer case; (2) a second stator element extending inwardly from theouter case which is spaced axially from the first stator element; (3) athird stator element spaced axially from the first stator element; (4)an array of stator vanes extending circumferentially about the flow pathat least one of which engages the first stator element at a first axiallocation and the second stator element at a second axial location, thestator vane being adapted to engage a duct at a third location locatedaxially between first location and the second location; (5) a ducthaving a plurality of duct segments at least one of which engages thevane at the third location and is trapped between one of said vanes ofthe array and the first stator element at the first location, the ducthaving a transition piece which extends between the first location andthe third location and radially inwardly of a portion of the vane toshield the vane from the working medium flow path, the transition piecehaving an extension which extends in cantilevered fashion into proximitywith the third stator element to shield the first stator element fromthe hot working medium gases.
 7. The stator assembly of claim 6 whereinthe first stator element and the second stator element are flangesattached to the outer case.
 8. The stator assembly as claimed in claim 6wherein the duct has a foot which is radially oriented and which extendsradially outwardly to engage the vane and the case at the first locationand wherein the foot of the duct, the extension of the duct, the outercase and the third stator element define a cooling chamber which isadjacent to the first stator element and which is adapted to be in flowcommunication with a source of cooling air.
 9. The stator assembly asclaimed in claim 8 wherein the vane is spaced radially inwardly from theouter case leaving a second cooling chamber therebetween which isbounded by the outer case and wherein a plurality of holescircumferentially spaced one from the other extend through the firststator element to place the second cooling chamber in flow communicationwith the first cooling chamber.
 10. The stator assembly as claimed inclaim 9 wherein the gas turbine engine has a high pressure turbine and alow pressure turbine, wherein the duct extends between the high pressureturbine and the low pressure turbine and wherein the transition piecehas an inner surface which is contoured for aerodynamic purposes toprovide a transition to the flow path boundary between the high pressureturbine and the low pressure turbine.
 11. The stator assembly as claimedin claim 10 wherein the vane has a foot which traps the foot of theduct, and further has an airfoil having a leading edge, a trailing edge,and a chordwise dimension L extending between the leading edge and thetrailing edge, wherein the distance from the airfoil edge nearest thevane foot to the end of the vane foot is a distance L' which is greaterthan or equal to L (L'≧L), wherein the length L' as compared withshorter lengths increases the resistance to the flow of heat from theedge region of the vane to the outer case at the first stator elementand the overall length of the vane as compared with overall lengths thatare shorter and increases the distance between the first and secondlocations to decrease the effect that differences in the radial growthof the case between the first location and the second location and theeffect that tolerance variations in the radial height of the first footof the vane and the radial height of the first foot of the duct have onthe rotation of the airfoil of the vane in the axial direction about thesecond location.
 12. For a gas turbine engine having a high pressureturbine which includes an array of rotor blades at the downstream end ofthe turbine, a low pressure turbine spaced axially from the highpressure turbine, and an annular flow path for working medium gaseswhich extends axially through the engine, the flow path including atransition region extending from the high pressure turbine to the lowpressure turbine, a stator assembly which bounds the working medium flowpath, which comprises:(I) an outer case extending circumferentiallyabout the working medium flow path having(1a) a first flange at a firstaxial location which extends inwardly from the outer case and which isadapted by a groove to trap a foot of a stator vane and a foot of aduct, the first flange having a plurality of cooling air slots extendingthrough the flange, (1b) a second flange at a second axial locationwhich extends inwardly from the outer case and which is spaced axiallyfrom the first flange, the second flange having a plurality of boltholes, each of which is circumferentially spaced from an adjacent hole;(II) an outer air seal assembly spaced axially from the first flangewhich includes(2a) a plurality of arcuate seal segments spaced radiallyfrom the array of rotor blades, and (2b) a plurality of arcuate ringsextending radially inwardly from the outer case to the arcuate sealsegments for supporting the seal segments from the outer case; (III) anarray of stator vanes extending circumferentially about the flow patheach of which is spaced radially from the outer case leaving acircumferentially extending chamber for cooling air therebetween andeach of which has(3a) a platform, (3b) at least one airfoil extendinginwardly from the platform across the working medium flow path, theairfoil having a leading edge, a trailing edge, and a chordwisedimension L extending between the leading edge and the trailing edge,(3c) a first foot which is trapped by the first flange on the outercase, the first foot extending from the leading edge of the airfoil fora length L' which is greater than or equal to the length L (L'≧L), (3d)a second foot adjacent to the second flange, the second foot having ahole which is aligned with an associated hole in the second flange toadapt the flange and the vane to receive a fastener for securing thevane to the second flange, and (3e) a groove which extendscircumferentially in the vane at a third location located axiallybetween the first location and the second location, the groove beingbounded by the platform and the first foot; (IV) a plurality offasteners each of which extends through an associated hole in the flangeand in the vane to attach the second vane foot second flange; (V) a ductformed of a plurality of duct segments spaced radially from the outercase to form a circumferentially extending chamber for cooling airtherebetween, each of the duct segments having(5a) a first foot which istrapped between an associated vane of the array and the first flange ofthe outer case at the first axial location, (5b) a second foot whichextends into the groove of the vane and is trapped by the vane at thethird location (5c) a transition piece radially inwardly of the firstfoot of the vane which extends axially between the first foot of theduct and the second foot of the duct, the transition piece having anextension which extends in cantilevered fashion from the first foot intoproximity with the outer air seal assembly, the extension having acurved nose extending axially over an adjacent outer air sealsegment;wherein the transition piece shields the first foot of the vanefrom the hot working medium gases and the length L' of the first footcauses a resistance in the foot to the flow of heat from the leadingedge region of the blade to the first flange which hampers the flow ofheat through the vane to the first flange, wherein the extension of thetransition piece shields the first flange from the hot working mediumgases and wherein the first flange has a plurality of holes extendingtherethrough to place the cooling air chamber between the outer case andthe duct in flow communication with the cooling air chamber between theouter case and the array of stator vanes such that the flow of coolingair through the first flange cools the flange and wherein the overalllength of the vane increases the distance between the first and secondlocations to decrease in comparison to a vane of a smaller overalllength the effect that differences in the radial growth of the casebetween the first location and the second location and the effect thattolerance variations in the radial height of the first foot of the vaneand the duct have on the rotation of the airfoil of the vane in theaxial direction about the second location.
 13. The invention as claimedin claim 12 wherein a seal member extends circumferentially about theinterior of the engine and extends axially from the first location andthe second location to provide a seal to the cooling air chamber betweenthe vane and the outer case.